Intertropical Convergence Zone
Intertropical Convergence Zone (ITCZ)
Many tropical areas experience only slight annual temperature variation, but very dramatic rainfall changes – often dividing their year into summer (dry season) and winter (wet season). Costa Rica is no exception, and the last few months of 2022 fully demonstrated the wrath of the ‘wet season’ – but all cannot be blamed on Tropical Storm Julia – because there is another culprit that snakes her soggy way around the globe, bringing storms, floods, mudslides and misery but also life-giving rains and reprieve to many areas around the tropical and sub-tropical world. This is the ITCZ, and with the wet season forecast to arrive in our Southern Pacific area between 23-26 of April 2023, this is a very fitting and timely post!
What is the ITCZ?
The ITCZ is also known as Monsoon Trough, Near-Equatorial Trough, The Doldrums or The Calms. It is a band of clouds, rain and storms that encircles the entire globe – essentially it is a permanent trough of low pressure. The ITCZ forms over the hottest parts of the world, roughly in line with where the sun is directly overhead (but lagging in distance by 1-2 months due to lagging heating of the ocean).
Not the entire stretch of the ITCZ is always ‘active’ – meaning showery or stormy. Large lengths of it may be dry due to factors such as dry air, variations in ocean water and air temperatures, local topography, or other conditions.
The ITCZ is a very important weather feature with implications for farmers, pilots, shipping, tourism, disaster management, and many others – for this reason the ITCZ is prominently displayed on weather charts. Below is a current surface analysis chart (for April 24, 2023) showing the ‘MONSOON TROF’, or ‘Monsoon Trough’. An ITCZ is labelled as ‘Monsoon Trough’ in areas of lower sea level pressure (there may be differences in where the ITCZ / Monsoon Trough’s location is presented on a weather chart based on height of observation and whether pressure or winds are used to denote the weather system). This label usually indicates an ‘active’ area of the ITCZ, meaning that clouds, rain and storms are present . We can see on the map below that today, April 24 2023, the ITCZ passes over the Osa Peninsula and then along the Costa Rica – Panama border.
Of course not all rain can be attributed to the ITCZ. Daytime heating, orographic lift, sea breezes, tropical storms and hurricanes, low pressure systems and fronts and other weather features give rise to showers and storms that contribute a fair amount of our annual precipitation. But when the ITCZ is sitting above your head, and it has been activated, you know the roof is getting a pounding!
How does the ITCZ form?
Where the land and ocean are the hottest, the air heats up and thus continually rises, forming rain and storm clouds. The rising air is replaced by winds that rush in to fill the void – these winds are known as ‘Trade Winds’. The Trade Winds rush into the ITCZ from, roughly, the north and the south and both replace the rising air and themselves cause more air movement in the upward direction when they collide head-on and are deflected up. Thus we have a circulation of air coming together from opposite directions, moving up, and then spilling back to where it came from.
Why is the ITCZ known as The Doldrums or The Calms when there are winds rushing into it?
When the northerly and southerly Trade Winds meet at the ITCZ they collide, push against each other and rise into the ITCZ. As the wind rise, the area below them remains calm and windless at the surface – therefore another, much older name for the ITCZ is The Doldrums (from the old-English word ‘dol’, which means ‘foolish’, ‘stupid’ and shares history with the current word ‘dull’, as in ‘monotonous’ and the verb ‘dull’ as in ‘depress’, ‘diminish’, ‘reduce’, ‘decrease’). Sail ships stuck in The Doldrums need to somehow make their way out or wait for the ITCZ to move past them so they can start to take advantage of the surrounding Trade Winds once again – a problem many centuries old.
On the weather chart below, you can clearly see the Doldrums. Areas of high and moderate wind are marked in green, yellow, orange and red. Calm and low winds are blue and purple. The Doldrums are the blue and purple areas, extending in a line that runs, roughly, from the bottom left to the top right of the chart – passing right through Costa Rica. This is the ITCZ, and the winds blowing into in from both sides are the Trade Winds – see how the Trade Winds converge into the blue and purple areas. Off the south-west coast of Costa Rica the calm area is quite extensive – any sailboat stuck here would be making a very slow passage, if moving at all. Hundreds of kilometers of very little wind.
How the ITCZ moves throughout the year
The ITCZ forms over the hottest parts of the land and sea and these hot areas move with the seasons. For this reason, in July the ITCZ sits over Mexico, Central America (including Costa Rica), Colombia and Venezuela. For January, the ITCZ shifts south, sitting over southern Colombia and northern Brazil – so it is in constant motion. Because land heats faster than water, where the ITCZ is situated over land it tends to be pulled more north in July and more south in January.
In Conclusion
Hopefully you now have a better understanding of a global weather pattern that impacts our life here (as I write this, the ITCZ stretches across the extreme southern territory of Costa Rica). So the next time someone complains about the rainy weather, be it at a party or a line in the checkout, you can feel confident to interject and endow them with a long and winding conversation about the ITCZ!
Just like most tropical and subtropical locations, our corner of Costa Rica is visited by tropical rain – it keeps the jungle lush and provides cooling relief during the hot afternoons, and it surely beats sub-freezing temperatures and blizzards! Come and experience the tropics – visit our RE/MAX We Sell Paradise website to view the hot properties in our local area: https://www.we-sell-paradise.com
